On the Control of Soil Heterogeneity, Peclet number and Spatially
Variable Diffusion over Unsaturated Transport
Physical properties of soils are ubiquitously heterogeneous. This
spatial variability has a profound, yet still partially understood,
impact on conservative transport. Moreover, molecular diffusion is often
a disregarded process that can have an important counter-intuitive
effect on transport: diffusion can prevent non-Fickian tailing by
mobilizing mass otherwise trapped in low velocity zones.
Here, we focus on macroscopically homogeneous soils presenting small
scale heterogeneity, as described by the Miller-Miller method. We then
analyze the dynamic control of soil heterogeneity, advection and
diffusion on conservative transport. We focus especially on the
importance of diffusion and of its tortuosity-dependent spatial
variability on the overall transport.
Our results indicate that high Peclet number systems are highly
sensitive to the degree of heterogeneity, which promotes non-Fickian
transport. Also, diffusion appears to have a profound impact on
transport, depending on both the degree of heterogeneity and the Peclet
number. For a high Peclet number and a very heterogeneous system,
diffusion leads to the counter-intuitive decrease of non-Fickian
macrodispersion described previously. This is not observed for a low
Peclet number due to the non-trivial impact of the spatial variability
in the diffusion coefficient, which appears to be a significant
controlling factor of transport by promoting or preventing the
accumulation of mass in low velocity zones.
Globally, this work (1) highlights the complex, synergistic effect of
soil heterogeneity, advective fluxes and diffusion on transport and (2),
alerts on potential upscaling challenges when the spatial variability of
such key processes cannot be properly described.